Recently, a technology of constructing a mobile voice over IP (mVoIP) network through interworking with an Internet protocol (IP) multimedia subsystem (IMS) network and a portable Internet, and of providing a portable Internet subscriber with an mVoIP-based call service (for example, a voice call service or a video call service) as an additional service, has been developed. The portable Internet may include, for example, a wireless broadband Internet (WiBro), a mobile Worldwide Interoperability for Microwave Access (WiMAX), a 3rd generation partnership project (3GPP) long term evolution (LTE), a 4th generation (4G) LTE, and the like.
An mVoIP is a technology of providing a VoIP service of a voice call level based on a data service of a portable Internet network. VoIP services may be classified into stationary VoIP services and mVoIP services, based on a mobility. In other words, a VoIP service based on a portable Internet, for example a mobile communication network, may be classified as an mVoIP service.
An instant message service may support an instant (real-time) text communication between at least two people, using a network, for example, the Internet. An instant messenger is a client to relay instant messages. The instant message service may enable real-time conversations and accordingly, differs from an electronic mail (e-mail) service. A conversation message may be transmitted every time people enter in a line and accordingly, the instant message service is similar to a telephone conversation rather than exchanging letters.
In addition, a group call service may enable a voice call between at least three participants at a time, and may include, for example, a conference call.